1. Field of the Invention
The present invention relates to flat display screens, and more specifically to so-called cathodoluminescence screens, the anode of which supports phosphor elements separated from one another by insulating areas, and likely to be energized by electron bombardment. This electron bombardment requires the biasing of the phosphor elements and can come from microtips, from layers of low extraction potential, or from a thermo-ionic source.
To simplify the present description, only color microtip screens will be considered hereafter, but it should be noted that the present invention relates, generally, to the various above-mentioned types of screens and the like.
2. Discussion of the Related Art
FIG. 1 shows the structure of a conventional flat color microtip screen.
Such a microtip screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which forms the screen surface.
The operating principle and a specific example of embodiment of a microtip screen are described, in particular, in U.S. Pat. No. 4,940,916 of the Commissariat a l'Energie Atomique.
Cathode 1 is generally organized in columns and is formed, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are made on a resistive layer 11 deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
This device uses the electric field created between cathode 1 and grid 3 to extract electrons from microtips 2. These electrons are then attracted by phosphor elements 7 of anode 5 if these elements are properly biased. In the case of a color screen, anode 5 is provided with alternate strips of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The strips are parallel to the cathode columns and are separated from one another by an insulator 8, generally silicon oxide (SiO.sub.2). Phosphor elements 7 are deposited on electrodes 9, formed of corresponding strips of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue strips are, in this example, alternately biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are alternately directed towards the phosphor elements 7 facing each of the colors.
Generally, the rows of grid 3 are sequentially biased to a potential on the order of 80 volts, while the strips of phosphor elements (for example, 7g in FIG. 1) to be energized are biased under a voltage on the order of 400 volts via the ITO strip on which these phosphor elements are deposited. The ITO strips, supporting the other strips of phosphor elements (for example, 7r and 7b in FIG. 1), are at a low or zero potential. The columns of cathode 1 are brought to respective potentials included between a maximum emission potential and a no emission potential (for example, respectively, 0 and 30 volts). The brightness of a color component of each of the pixels in a line is thus determined.
The choice of the values of the biasing potentials is linked to the characteristics of phosphor elements 7 and of microtips 2. Conventionally, below a potential difference of 50 volts between the cathode and the grid, there is no electron emission, and the maximum emission used corresponds to a potential difference of 80 volts.
A disadvantage of conventional screens is that they suffer from a low lifetime, that is, after a relatively short operating time (on the order of one hundred hours), the screen brightness considerably decreases and destructive phenomena due to the formation of arcs between the screen cathode and anode even sometimes occur.
Further, after a certain operating time, the color appears to vary and no longer corresponds to the screen control orders. This phenomenon will be referred to herein as the "color drift". In practice, this means that one at least of the strips of phosphor material adjacent to the biased strips starts exhibiting a luminescence.
A first known technique to avoid this phenomenon of parasitic luminescence consists of separating, by short time intervals, the biasings of the anode strips between two successive color sub-frames, and applying a negative voltage pulse on the strip just biased before positively biasing the following anode strip to be energized.
However, this technique has the disadvantage of complicating the circuits providing the anode supply voltages, which are voltages of high values (some hundred volts), and of being prejudicial to the screen brightness.
A second known technique consists of depositing, on the insulating strips separating the phosphor strips, a conductive layer biased to a negative or null potential. Such a technique is described, for example, in patent EP-A-0635865. The function of the conductive layer then is to prevent the emission of secondary electrons by the SiO.sub.2 insulating layer, and to create a positive charge area between two strips of phosphor elements.
However, a disadvantage of this technique is that, in order not to bear prejudice to the insulation between anode strips, the insulating strips have to be thick (on the order of 50 .mu.m) so that the deposited phosphor elements (over a thickness on the order of 10 .mu.m) between these strips are buried with respect to the conductive layer. Another disadvantage is that the implementation of this technique lengthens the duration of manufacturing. Indeed, to be accurate, the etching of the SiO.sub.2 layer must be performed by plasma, which is particularly long for such a thickness.
Another disadvantage is that the thickness of the insulating strips must be chosen according to the resistivity of the phosphor elements while this resistivity can be different from one color to another.
Further, the phosphor elements are generally deposited by serigraphy. Now, the alignment of the serigraphy mask with the etching pattern is, in practice, imperfect, so that phosphor elements often extend slightly beyond the holes of the SiO.sub.2 layer. In such a case, the insulation is no longer respected, since these over-extensions are performed over the conductive layer.